The present invention generally relates to a soldering process and apparatus dealing with printed circuit boards having mounted thereon chip type elements or components without leads, such as resistors, capacitors or the like.
For soldering of the chip type components referred to above, there conventionally has been employed a spouting type soldering system in which soldering is effected by spouting or applying molten solder onto a soldering surface of a printed circuit board during movement of the printed circuit board. Such spouting type soldering system broadly is divided into two systems according to configurations of the spouting solder waves, i.e. one system referred to as a dual or double direction solder wave spouting type in which the solder wave is formed into an approximately symmetrical configuration with respect to a spouting port or nozzle for the molten solder, and another system referred to as a single direction solder wave spouting type which provides a solder wave flowing in a direction counter to the direction of movement of the printed circuit board.
Both of these known soldering systems, however, have serious disadvantages inherent therein. More specifically, in the former double direction solder wave spouting type system, a solder wave 2 is spouted from a nozzle 1 in a symmetrical configuration as shown in FIG. 1, while a printed circuit board 4, on which chip type components 3 are mounted, is moved from left to right as indicated by an arrow in FIG. 1. In the above arrangement, however, when the circuit board 4 leaves or exits from the solder wave 2, molten solder of the solder wave 2 tends to flow down in the same direction as the direction of movement of the printed circuit board 4, with excessive solder adhering to the chip type components 3, and thus, as shown in FIG. 2, excessive solder paddings, i.e. fillets 9 higher in height than the chip type components 3, are undesirably formed between electrode portions 6 of the chip type components 3 mounted on the printed circuit board 4 by a bonding agent 5 and conductor portions 8 of the circuit board 4 surrounded by a solder resist layer 7. As is seen from the graph of FIG. 3, when the finished printed circuit board 4 is subjected to deformation (bending) due to application of an external force or heat, cracks are readily formed within the chip type components 3, which are rigidly connected to the circuit board 4 by the excessive solder paddings 9, leading to possible deterioration of the characteristics thereof, and thus giving rise to malfunctions of the circuit when the circuit board 4 is incorporated into electronic appliances or other products.
On the other hand, in the single direction solder wave spouting type proposed to prevent the formation of the excessive solder paddings referred to above, as shown in FIG. 4, a single solder wave 11 flows from a nozzle 10 in a direction counter to the direction of movement of the printed circuit board 4, while the circuit board 4 passes through the solder wave 11 along a path inclined upwardly with respect to the direction of movement thereof. In the above arrangement, when the circuit board 4 exits from the solder wave 11, molten solder of the solder wave 11 flows down in a direction counter to the direction of the circuit board movement so as to be dragged into solder wave 11, without forming excessive paddings or fillets. In the above arrangement, however, since the solder wave 11 is directed only to one side of the chip type components 3, i.e. in a direction from right to left as shown in FIG. 4, although a normal solder padding 13 not exceeding the height of the chip type component 3 is obtained at the forward portion 12 of each component 3 with respect to the advancing direction of the circuit board 4 as shown in FIG. 5, gas of flux evaporated during the soldering operation undesirably stays at the rear portion 14 of the component 3 without being removed therefrom, and thus no molten solder adheres to such rear portion 14, thereby inevitably requiring correction by further manual solder padding work at a later stage of processing.
As is seen from the foregoing description, in both of the conventional spouting type soldering systems, faulty soldering tends to take place, thus undesirably necessitating corrections by manual operations, in which case there occurs the possibility of a still more serious problem in that the chip type component is liable to be damaged by local heat applied thereto by a soldering iron employed for the repairing operation. Therefore, there has been a demand to provide a soldering process for chip type components in which faulty soldering is eliminated, without the necessity for corrections at later stages of processing.
In the course of repeated research for ways to eliminate the necessity of corrections as described above through removal of faulty soldering, the present inventors have also made investigations into the method as disclosed, for example, in U.S. Pat. No. 4,101,066 in which a printed circuit board is brought into contact with two independent solder waves by being passed therethrough. The second solder wave flows in the same direction as that of the movement of the printed circuit board, but as a result it has been found that even the prior art process of U.S. Pat. No. 4,101,066 is not fully effective for the solution of such soldering defects as bridges or short circuits, etc., due to the large size of the solder paddings adhering to the chip type components.